In recent years, wireless power supply systems for charging battery-equipped devices or for supplying power to mobile devices, such as a tablet terminal, by utilizing electromagnetic induction between coils, are more and more widely used. In such a wireless power supply system, the wireless power receiver apparatus may request the wireless power transmitter apparatus to increase or decrease the amount of power to be supplied, or the like. Therefore, the wireless power receiver apparatus communicates to the wireless power transmitter apparatus using the load modulation scheme in which the wireless power receiver apparatus changes the magnitude of a load (e.g., power consumption of the wireless power receiver apparatus, which is variable by changing a load resistance), for example, by a switch device provided in the wireless power receiver apparatus, and notifies the wireless power transmitter apparatus of a change in the load.
For example, the power transmitter apparatus disclosed in the Patent Document 1 is known as a wireless power transmitter apparatus using the load modulation scheme. The power transmitter apparatus of the Patent Document 1 is a power transmitter apparatus characterized by detecting an electrical potential at one end of a power transmitting coil, limiting the detected electrical potential to be equal to or larger than a ground potential, and thus, extracting and demodulating a signal sent from a power receiver apparatus to the power transmitter apparatus. In the wireless power supply systems, it is necessary for safe power supply to maintain such a state that the power transmitter apparatus can receive signals from the power receiver apparatus and always demodulate the signals correctly.